The basic building blocks of thin-film bulk-acoustic wave filters are BAW resonators using either Film-Bulk-Acoustic-wave-Resonator (FBAR) or Solidly-mounted-Bulk-Acoustic-wave-Resonator (SBAR or SMR) technology. Low insertion-loss in the pass-band of such a filter is an essential requirement. Typically 2 to 3 dB insertion loss is achieved, while less than 1 dB is preferred in many specifications.
In recent years investigations have been made with a view to improving the performance of BAW devices.
J. Kaitila, et al, “Spurious Resonance Free Bulk Acoustic Wave Resonators”, IEEE Ultrasonics Symposium, Honolulu, p. 84, 2003 discloses a method for reducing unwanted standing waves by employing a frame-region round each resonator. It is said that it reduces energy loss to some extent but there has been no adequate explanation of why such a construction should reduce loss.
US 2002/0030424 A1 discloses suppressing the occurrence of spurious modes due to an inharmonic mode generated in a high frequency piezoelectric resonator by a device structure comprising an AT cut quartz crystal plate having top and bottom surfaces. A top excitation electrode, termed the main electrode, is provided on a predetermined area of the top surface and a bottom excitation electrode is provided over the entire area of the bottom surface. A second electrode is provided on the top surface surrounding the main electrode but leaving a gap between the juxtaposed edges of the main and second electrodes. The second electrode is provided for suppressing spurious modes. In some of the embodiments described the materials of the main and second electrodes are different with the density of the material, for example silver, comprising the second electrode being lower than the density of the material, for example gold, comprising the main electrode. Additionally the thickness of the second electrode is greater than that of the main electrode in order to restrict the object waves of the energy-trapped mode to only the main vibration alone. In the case of using gold and silver for the main and second electrodes, respectively, the thickness of the second electrode is approximately twice that of the main electrode. Further it is necessary to set the cut-off frequencies of the main and second electrodes so that the cut-off frequency (f3) of the second electrode is higher than the cut-off frequency (f1) of the main electrode and the cut-off frequency (f2) of the gap is higher than f3, viz, f1<f3<f2.
Earlier research, R. F. Milsom, et al “Combined acoustic-EM simulation of thin-film bulk acoustic wave filters”, IEEE Ultrasonics Symposium, Munich, p. 963, 2002, established that the dominant loss mechanism in thin-film BAW resonators is acoustic in origin, except over a very narrow range of frequencies near the resonance frequency fr (frequency of maximum admittance) where electrical resistance is the dominant source of loss. It is also clear, from the observation that the unexplained losses varied strongly with frequency, that viscosity and other material-related mechanisms within the thin-film layers could not be the primary cause. For example, the acoustic loss at anti-resonance fa is much greater than at resonance, where these two frequencies differ only by about 3%. From this strong frequency-dependence it was concluded that the likely cause was the unwanted radiation of travelling acoustic wave energy away from each resonator, accompanying the wanted thickness extensional (TE) mode standing-wave energy. The travelling acoustic waves are Lamb waves in the case of a FBAR and surface acoustic waves and leaky waves in the case of a SBAR. However, prior to the development of the new 2D model, the precise mechanism was not identified or quantified, and therefore no effective remedy could be introduced.